The slice is being converted internally into a list: ($x..$x), and then that list is being evaluated in a scalar context, which means that it is not a list, but an expression with multiple comma operators. And the comma operator yields its right-hand side.
Interestingly, defined works the same way, so defined(@x[2..7]) is the same as defined($x). So at least when you undef a hash slice, testing it for definedness works as expected, even though several of the values are defined.
I note again, you cannot provide a list to undef or defined. You get a compiler error. For defined, you can say
and it does the same as it does for a slice. But you can't use scalar() with undef, because scalar doesn't yield lvalues.defined scalar($x,$x) or warn "not defined!";
Caution: Contents may have been coded under pressure.